The Champions Tour said Sindelar's condition had improved but doctors at Sonoma Valley Hospital wanted to keep the 51-year-old for further observation.
Sindelar withdrew Saturday and was taken to the hospital for dizziness and shortness of breath after playing the 204-yard, par-3 fourth hole. Sindelar rested in between holes and tried to play the fifth before withdrawing. Medical personnel attended to Sindelar at the course before he was taken to the hospital for additional tests.
Sindelar, a seven-time winner on the PGA Tour, was 7 under for the tournament before withdrawing.